An electrical transmission line from a generator to consumers must be protected against insulation failure, or overload, by at least one circuit breaker. It is a mechanical switching device for making, carrying and breaking an electrical contact, comprising a pair of conductor terminals and a bridging member opening and closing the gap therebetween. For high voltage AC-, or high amperage DC-currents, circuit breakers have been designed to operate within narrow time-limits, e.g. within a few cycles, i.e. the period of less than 0.1 second, thereby minimizing damage to generator and consumer-devices alike.
Since it is not possible to interrupt a high voltage, or a large electrical current instantaneously, attention is focussed on: (a) minimizing the mass (inertia) of said bridging member, as is the case in the simple fuse; (b) maximizing the force for moving said member, e.g. by compressed springs or gases, or by electromagnetic forces generated by the fault within the circuit; and (c) extinguishing the sparks (ionized gas and metal vapor) between terminals and bridging member. The latter is achieved with either liquid-filled, or gas-blast circuit breakers, wherein the sparks are removed by vaporization and recirculation of an insulating liquid, such as mineral oil, or diluted with insulating gas, such as air or sulfur hexafluoride (SF.sub.6), thereby spreading the sparks over an enlarged area (arc chute).